The objective of this project is to gain a better understanding of the molecular basis for leukemia development by utilizing retrovirus-based mouse model systems. We are currently studying the Friend spleen focus- forming virus (SFFV), which causes an acute erythroleukemia in certain strains of mice. Due to expression of the unique envelope glycoprotein encoded by SFFV, virus- infected erythroid cells are able to proliferate and differentiate in the absence of erythropoietin (Epo), the normal regulator of erythropoiesis. In an attempt to understand how SFFV alters the growth and differentiation of erythroid cells, we have been studying signal transduction pathways known to be activated by Epo to determine if SFFV exerts its biological effects by constitutively activating any of these pathways. Our studies indicate that infection of erythroid cells with SFFV results in the constitutive activation of components of both the Jak-Stat and Raf-1/MAP kinase signal transduction pathways. Studies utilizing a mutant of SFFV that failed to induce Epo-independent differentiation, as well as SFFV-induced erythroleukemia cell lines that are blocked in differentiation, suggest that activation of the Jak-Stat pathway may be required for Epo-independent differentiation of SFFV- infected erythroid cells. SFFV infection of erythroid cells from mice carrying the Fv2rr gene, which confers resistance to SFFV-induced erythroleukemia, fails to result in the activation of the Jak-Stat pathway in the absence of Epo, indicating that activation of this pathway is important for the induction of erythroleukemia by SFFV. Studies to examine the effects of SFFV on the Raf-1/MAP kinase pathway in these mice are in progress. Finally, we have been examining SFFV- infected erythroid cells for the expression of immediate early genes that are known to be activated by Epo. SFFV-infected cells showed constitutive activation of a number of immediate early genes induced by Epo in uninfected cells. Interestingly, several immediate early genes that are thought to have a negative role in Epo-induced signal transduction failed to be activated in SFFV-infected cells, even in the presence of Epo. Thus, SFFV infection of erythroid cells leads to the constitutive activation of several signal transduction pathways, resulting in the deregulation of genes involved in the activation as well as inhibition of erythroid cell growth.